Die casting machine and control method thereof

ABSTRACT

Provided is a die casting machine and die casting machine control method that are able to change the operation of a plunger at an appropriate timing. A die casting machine operates a plunger such that the plunger moves forward, detects that molten metal extruded from a sleeve by the forward movement of the plunger has arrived at a cavity, determines a deceleration starting position and a intensification starting position for changing the operation of the plunger, using, as a reference (reference position Ls), the position of the plunger 15 at the time when it is detected that molten metal has arrived at the cavity, and, when the plunger moves to the deceleration starting position and intensification starting position, changes the operation of the plunger to operations corresponding to the respective positions.

TECHNICAL FIELD

The present invention relates to a die casting machine and a method for controlling a die casting machine.

BACKGROUND ART

Japanese Unexamined Patent Application Publication No. 2008-126294 discloses a conventional die casting machine. This die casting machine has a low-speed injection process and high-speed injection process, in which injection operations are performed, and an intensification process, in which an intensification operation is performed. The die casting machine changes the low-speed injection process to the high-speed injection process at the timing when the forward movement position of an injection plunger has moved forward by a distance set in the low-speed injection process. Upon detecting that the injection plunger has moved to a predetermined forward movement position, the die casting machine changes speed feedback control along the position axis to pressure feedback control along the time axis.

SUMMARY OF INVENTION

The above die casting machine injects molten metal poured into a sleeve, into a cavity of molds using a plunger so that the cavity is filled with the molten metal. However, if the molten metal overflows the sleeve for some reason or adheres to the inside of a ladle when poured into the sleeve, the amount of molten metal in the sleeve varies. Accordingly, if the operation of the plunger is changed using, as a reference, the absolute position of the plunger with the amount of molten metal varying, the amount of molten metal in the cavity also varies. That is, the operation of the plunger may be changed at an inappropriate timing, resulting in destabilization of the quality of the molded product.

In view of the foregoing, an object of the present invention is to provide a die casting machine and die casting machine control method that are able to change the operation of a plunger at an appropriate timing.

To achieve the above object, a die casting machine according to an aspect of the present invention includes a sleeve into which molten metal to be injected to a cavity of a mold is poured, a plunger housed in the sleeve, an arrival detector configured to detect that molten metal extruded from the sleeve by a forward movement of the plunger has arrived at the cavity, an operation change position determination unit configured to determine an operation change position for changing an operation of the plunger, using, as a reference, a position of the plunger at the time when the arrival detector detects that molten metal has arrived at the cavity, and an operation controller configured to operate the plunger such that the plunger moves forward and to, when the plunger moves to the operation change position, change the operation of the plunger.

Preferably, the die casting machine of the present invention further includes a piston connected to the plunger and configured to move by hydraulic pressure and a cylinder housing the piston, the operation controller operates the plunger by controlling supply of hydraulic oil to a rear oil chamber of the cylinder and discharge of hydraulic oil from a front oil chamber of the cylinder, and the arrival detector detects that molten metal has arrived at the cavity, on the basis of a change in hydraulic pressure of the front oil chamber.

In the present invention, the arrival detector preferably detects that molten metal has arrived at the cavity, on the basis of a change in speed of the plunger.

In the present invention, the arrival detector preferably detects that molten metal has arrived at the cavity, within a predetermined monitoring range set with respect to the position of the plunger or the time during which the plunger is operating.

In the present invention, preferably, the operation change position is a deceleration starting position, and the operation controller decelerates the plunger when the plunger moves to the deceleration starting position.

In the present invention, preferably, the operation change position is an intensification starting position, and the operation controller changes the operation of the plunger to an intensification operation when the plunger moves to the intensification position.

To achieve the above object, another aspect of the present invention provides a method for controlling a die casting machine, the die casting machine including a sleeve into which molten metal to be injected to a cavity of a mold is poured and a plunger housed in the sleeve. The method includes operating the plunger such that the plunger moves forward, detecting that molten metal extruded from the sleeve by a forward movement of the plunger has arrived at the cavity, determining an operation change position for changing an operation of the plunger, using, as a reference, a position of the plunger at the time when it is detected that molten metal has arrived at the cavity, and when the plunger moves to the operation change position, changing the operation of the plunger.

According to the present invention, the plunger is operated such that the plunger moves forward, it is detected that the molten metal extruded from the sleeve by the forward movement of the plunger has arrived at the cavity of the mold, the operation change position for changing the operation of the plunger is determined using, as a reference, the position of the plunger at the time when it is detected* that the molten metal has arrived at the cavity, and the operation of the plunger is changed when the plunger moves to the operation change position. Since the volume of the cavity is constant, the amount of molten metal to be extruded after the molten metal has arrived at the cavity is also predetermined. For this reason, the position of the plunger at the time when the molten metal has arrived at the cavity is defined as the reference position, and the operation change position corresponding to the volume of the cavity is determined relative to the reference position. Thus, the operation of the plunger can be changed at an appropriate timing regardless of variations in the amount of molten metal in the sleeve. As a result, the quality of the product can be effectively stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a schematic configuration of a die casting machine according to an embodiment of the present invention;

FIG. 2 is a sectional view of a die plate and its vicinity of the die casting machine in FIG. 1;

FIG. 3 is a diagram showing function blocks of a controller of the die casting machine in FIG. 1;

FIG. 4 is a flowchart showing an example of a process according to the present invention performed by the controller in FIG. 3;

FIG. 5 is an enlarged sectional view of a cavity and its vicinity of molds and is a drawing showing a state in which molten metal has been poured into a sleeve;

FIG. 6 is an enlarged sectional view of the cavity and its vicinity of the molds and is a drawing showing a state in which a plunger has moved to a high-speed forward movement starting position;

FIG. 7 is an enlarged sectional view of the cavity and its vicinity of the molds and is a drawing showing a state in which the molten metal has arrived at the cavity;

FIG. 8 is an enlarged sectional view of the cavity and its vicinity of the molds and is a drawing showing a state in which the plunger has moved to a deceleration starting position;

FIG. 9 is an enlarged sectional view of the cavity and its vicinity of the molds and is a drawing showing a state in which the plunger has moved to an intensification starting position;

FIG. 10 is a graph showing an example of changes in the hydraulic pressures of front and rear oil chambers of a cylinder and changes in the speed of the plunger in injection processs and an intensification process (a configuration that detects the molten metal has arrived at the cavity, on the basis of a change in the hydraulic pressure of the front oil chamber) ; and

FIG. 11 is a graph showing an example of changes in the hydraulic pressures of the front and rear oil chambers of the cylinder and changes in the speed of the plunger in the injection process and intensification process (a configuration that detects the molten metal has arrived at the cavity, on the basis of a change in the speed of the plunger).

DESCRIPTION OF EMBODIMENTS

Now, a die casting machine according to an embodiment of the present invention will be described with reference to FIGS. 1 to 10.

FIG. 1 is a drawing showing a schematic configuration of the die casting machine according to the embodiment of the present invention. FIG. 2 is a sectional view of a die plate and its vicinity of the die casting machine in FIG. 1. FIG. 3 is a diagram showing function blocks of a controller of the die casting machine in FIG. 1. FIG. 4 is a flowchart showing an example of a process according to the present invention performed by the controller in FIG. 3. FIGS. 5 to 9 are enlarged sectional views of a cavity and its vicinity of molds and are drawings showing (1) a state in which molten metal has been poured into a sleeve, (2) a state in which a plunger has moved to a high-speed forward movement starting position, (3) a state in which the molten metal has arrived at the cavity, (4) a state in which the plunger has moved to a deceleration starting position, and (5) a state in which the plunger has moved to an intensification starting position, respectively. FIG. 10 is a graph schematically showing an example of changes in the hydraulic pressures of front and rear oil chambers of a cylinder and changes in the speed of the plunger in injection processs and an intensification process (a configuration that detects the molten metal has arrived at the cavity, on the basis of a change in the hydraulic pressure of the front oil chamber). In the present specification, the left side of FIGS. 1, 2, and 5 to 9 is defined as the front side, and the right side thereof is defined as the rear side.

As shown in FIG. 1, a die casting machine 1 according to the present embodiment includes a piston 11 that moves by hydraulic pressure and a cylinder 12 that houses the piston 11 such that the piston 11 can move in the front-rear direction. The cylinder 12 has a rear oil chamber 13 and a front oil chamber 14 separated by the piston 11. A plunger 15 is connected to the piston 11. The plunger 15 includes a rod 16 that extends forward from the front surface 11 a of the piston 11 and a plunger tip 17 disposed on an end of the rod 16. The plunger 15 is inserted into a sleeve 33 (FIG. 2) of a fixed die plate 31 so as to be able to move back and forth.

A drive transmission plate 18 is disposed behind the cylinder 12. The drive transmission plate 18 is moved back and forth by an electric servo motor 19 and a drive transmission mechanism 20 including a drive transmission gear, a ball screw mechanism, and the like. Once it is moved forward, the drive transmission plate 18 presses and moves forward the cylinder 12. The drive transmission plate 18, electric servo motor 19, and drive transmission mechanism 20 form a motor-driven intensification mechanism. Instead of such a motor-driven intensification mechanism, there may be used a hydraulic intensification mechanism that presses and moves forward the cylinder 12 by hydraulic pressure.

The die casting machine 1 includes an accumulator 21 serving as an hydraulic pressure supplier that supplies hydraulic oil to the rear oil chamber 13 of the cylinder 12. The accumulator 21 and the rear oil chamber 13 of the cylinder 12 are connected through an oil passage a. A supply valve 22 is disposed on the oil passage a. The supply valve 22 establishes and blocks communication between the accumulator 21 and rear oil chamber 13. In the present embodiment, the supply valve 22 consists of an electromagnetic valve that drives a valve using a solenoid. The supply valve 22 may be a valve of a type other than an electromagnetic valve, for example, one that drives a valve using a motor.

The front oil chamber 14 of the cylinder 12 and a tank 23 that stores hydraulic oil are connected through an oil passage b. A servo valve 24 is disposed on the oil passage b. The servo valve 24 adjusts the amount of hydraulic oil discharged from the front oil chamber 14 to the tank 23, on the basis of the degree of opening. In the present embodiment, the speed at which the plunger 15 moves forward, or the like is controlled by adjusting the amount of hydraulic oil discharged from the front oil chamber 14 using the servo valve 24 (meter-out control) Note that the supply valve 22 may be a servo valve and the speed at which the plunger 15 moves forward, or the like may be controlled by adjusting the amount of hydraulic oil supplied from the accumulator 21 to the rear oil chamber 13 using the servo valve (meter-in control).

The die casting machine 1 also includes an hydraulic pressure sensor 25 that outputs a signal corresponding to the hydraulic pressure Pf of the front oil chamber 14 of the cylinder 12 and a position sensor 26 that outputs a signal corresponding to the position L of the plunger 15. The position sensor 26 may be a known position sensor, such as optical, magnetic, or magnetostriction type.

As shown in FIG. 2, the die casting machine also 1 includes a fixed die plate 31 having a fixed mold K1 mounted thereon and a movable die plate 32 having a moving mold K2 mounted thereon. The fixed die plate 31 includes the cylindrical sleeve 33 that communicates with an runner R of the fixed mold K1. The sleeve 33 has an inlet 34 formed in an upper portion thereof, and molten metal M is poured into the sleeve 33 through the inlet 34 using a ladle (not shown). The die casting machine 1 opens and closes the fixed mold K1 and moving mold K2 by moving the movable die plate 32 back and forth with respect to the fixed die plate 31 using a mold closure drive unit (not shown).

The closed fixed mold K1 and moving mold K2 form therewithin the runner R communicating with the sleeve 33, a cavity C communicating with the runner R, and an overflow portion communicating with the cavity C. The cavity C-side end of the runner R has smaller diameters at positions closer to the cavity C and communicates with a gate G open to the cavity C. Thus, the molten metal M passes through the gate G when flowing from the runner R into the cavity C, increasing the flow resistance thereof.

The die casting machine 1 also includes a controller 40 that controls the overall operation. The controller 40 includes, for example, a microcomputer for built-in devices that includes a CPU, a ROM, a RAM, an EEPROM, and I/O interfaces. The controller 40 controls the operation of the accumulator 21, supply valve 22, and servo valve 24. The controller 40 also detects the hydraulic pressure Pf of the front oil chamber 14 on the basis of a signal outputted from the hydraulic pressure sensor 25. The controller 40 also detects the position L and the speed V of the plunger 15 on the basis of a signal outputted from the position sensor 26. The controller 40 also controls the operation of the drive units in the processs, such as a mold closing process, a metal pouring process, injection processs (low-speed injection process and high-speed injection process), an intensification process, a mold opening process, and a product ejection process.

The CPU of the controller 40 functions as a hydraulic pressure detector 41, a position/speed detector 42, an arrival detector 43, an operation change position determination unit 44, and an operation controller 45 by executing a control program stored in the ROM.

The hydraulic pressure detector 41 detects the hydraulic pressure Pf of the front oil chamber 14 on the basis of a signal outputted from the hydraulic pressure sensor 25. The position/speed detector 42 detects the position L and speed V of the plunger 15 on the basis of a signal outputted from the position sensor 26.

The arrival detector 43 detects that the molten metal M has arrived at the cavity C (that is, the top of the molten metal M has passed through the gate G) by the forward movement of the plunger 15.

Specifically, the arrival detector 43 monitors the hydraulic pressure Pf of the front oil chamber 14 detected by the hydraulic pressure detector 41 while the position L of the plunger 15 is in a predetermined monitoring range Lw. The monitoring range Lw is set according to the system or molds, for example, is set to a range from a position (starting position La) ahead of a position L0 (not shown) by A mm, the position L0 being a position in which the plunger 15 starts to move forward, to a position (ending position Lb) ahead of the starting position La by B mm. Note that the monitoring range Lw does not have to be set. When the arrival detector 43 detects that the hydraulic pressure Pf of the front oil chamber 14 has become a lower value than the no-load-time hydraulic pressure P0 of the front oil chamber 14 by a predetermined percentage α (Pf≤P0×(1−α), 0<α<100%, e.g., α=20%) or more, it determines that the molten metal M has arrived at the cavity C. The reason is as follows: when the top of the molten metal M passes through the gate G, the flow resistance increases and thus a load (pressure) is applied to the plunger 15; and the hydraulic pressure Pf of the front oil chamber 14 is reduced so that the hydraulic pressure Pf and the hydraulic pressure Pb of the rear oil chamber 13 are balanced in the load-applied plunger 15.

Instead of setting the monitoring range Lw with respect to the position of the plunger 15, the arrival detector 43 may set a monitoring range Tw with respect to the time T during which the plunger 15 is operating. The monitoring range Tw is set according to the system or mold, for example, is set to D sec (until a time Tb) after a time Ta when C sec elapses after a reference time T0 when the plunger 15 starts to move forward in the low-speed injection process. As seen above, the monitoring range Lw or monitoring range Tw in which the hydraulic pressure Pf of the front oil chamber 14 is monitored is set with respect to the position L of the plunger 15 or the time during which the plunger 15 is operating. Thus, the hydraulic pressure Pf can be monitored while limiting the range to the position or time in which the hydraulic pressure Pf of the front oil chamber 14 is reduced with a high probability. As a result, the arrival detector 43 can be prevented from falsely detecting that the molten metal M has arrived at the cavity C.

The no-load-time hydraulic pressure P0 of the front oil chamber 14 is calculated on the basis of the hydraulic pressure set for the accumulator 21. The hydraulic pressure Pb of the rear oil chamber 13 communicating with the accumulator 21 is set to the same hydraulic pressure as that set for the accumulator 21. The no-load-time hydraulic pressure P0 of the front oil chamber 14, which is that when no load is being applied to the plunger 15, can be calculated by multiplying the hydraulic pressure Pb of the rear oil chamber 13 by the area ratio between the rear surface 11 b and front surface 11 a of the piston 11 [no-load-time hydraulic pressure P0 of front oil chamber 14=hydraulic pressure Pb of rear oil chamber 13×(area Sb of rear surface 11 b/area Sa of front surface 11 a of piston 11)].

The operation change position determination unit 44 determines the deceleration starting position L2 and intensification starting position L3 for changing the operation of the plunger 15, using, as a reference, the position L of the plunger 15 at the time when the arrival detector 43 detects that the molten metal M has arrived at the cavity C. The deceleration starting position L2 and intensification starting position L3 are operation change positions.

Specifically, when the arrival detector 43 detects that the molten metal M has arrived at the cavity C, the operation change position determination unit 44 acquires the position L (i.e., the reference position Ls) of the plunger 15 detected by the position/speed detector 42 at that time (time Ts). Then, on the basis of the inner diameter of the sleeve 33, the volume of the cavity C, and the like, the operation change position determination unit 44 determines the deceleration starting position L2 and the intensification starting position L3 relative to the reference position Ls, the deceleration starting position L2 being a position in which the plunger 15 is decelerated in the high-speed injection process, the intensification starting position L3 being a position in which speed control over the operation of the plunger 15 is changed to pressure control and a intensification operation is started.

The deceleration starting position L2 is, for example, a position in which the entire cavity C is filled with the molten metal M. If the amount of molten metal M extruded from the sleeve 33 becomes equal to the volume of the cavity C when the plunger 15 moves from the reference position Ls by the distance D2, the deceleration starting position L2 is set to a position ahead of the reference position Ls by the distance D2.

The intensification starting position L3 is, for example, a position in which the entire cavity C and overflow portion O are filled with the molten metal M. If the amount of molten metal M extruded from the sleeve 33 becomes equal to the total volume of the cavity C and overflow portion O when the plunger 15 moves from the reference position Ls by the distance D3, the intensification starting position L3 is set to a position ahead of the reference position Ls by the distance D3.

The operation controller 45 operates the plunger 15 such that the plunger 15 moves forward. Also, when the plunger 15 moves to the high-speed forward movement starting position L1, deceleration starting position L2, or intensification starting position L3, the operation controller 45 changes the operation of the plunger 15 to an operation corresponding to that position. The high-speed forward movement starting position L1 is preset.

Specifically, the operation controller 45 operates the plunger 15 by setting the hydraulic pressure of the accumulator 21, opening the supply valve 22, and adjusting the degree of opening of the servo valve 24 to control the supply of the hydraulic oil to the rear oil chamber 13 of the cylinder 12 and the discharge of the hydraulic oil from the front oil chamber 14 of the cylinder 12.

As a series of product molding operations, the die casting machine 1 first closes the fixed mold K1 and moving mold K2 by driving the movable die plate 32 (mold closing process). Then, the molten metal M is poured into the sleeve 33 of the fixed die plate 31 (metal pouring process). Then, the die casting machine 1 moves the piston 11 forward by increasing the hydraulic pressure of the hydraulic oil in the accumulator 21, opening the supply valve 22 to supply the hydraulic oil to the rear oil chamber 13 of the cylinder 12, and opening the servo valve 24 to discharge the hydraulic oil from the front oil chamber 14. At this time, the die casting machine 1 adjusts the amount of hydraulic oil to be discharged, on the basis of the degree of opening of the servo valve 24, moves the piston 11 forward at low speed and then at high speed, injects the molten metal M from the sleeve 33 into the cavity C using the plunger 15 so that the cavity is filled with the molten metal (low-speed injection process, high-speed injection process). The die casting machine 1 also presses the cylinder 12 (i.e., piston 11) by driving the electric servomotor 19 to move the drive transmission plate 18 forward (intensification process). The die casting machine 1 then opens the fixed mold K1 and moving mold K2 by driving the movable die plate 32 (mold opening process) and ejects a product from the cavity C (product ejection process).

Next, as a method for controlling the die casting machine 1 of the present embodiment, an example of a process according to the present invention performed by the controller 40 will be described with reference to a flowchart of FIG. 4 and FIGS. 5 to 10. This process is performed in the low-speed injection process and high-speed injection process.

As shown in FIG. 5, after the molten metal is poured into the sleeve 33 in the metal pouring process, the controller 40 starts the low-speed injection process to operate the plunger such that the plunger moves forward at low speed (S110). Specifically, the controller 40 increases the pressure of the hydraulic oil in the accumulator 21 by setting the hydraulic pressure of the accumulator 21, then supplies the hydraulic oil to the rear oil chamber 13 of the cylinder 12 by opening the supply valve 22, and then discharges the hydraulic oil from the front oil chamber 14 by adjusting the degree of opening of the servo valve 24, Thus, the plunger 15 connected to the piston 11 moves forward from the position L0 as the starting point at low speed (e.g., 0.5 m/sec).

The controller 40 then waits until the plunger 15 moves to the high-speed forward movement starting position L1 (N in S120). When the plunger 15 moves to the high-speed forward movement starting position L1, as shown in FIG. 6 (Y in S120), the controller 40 starts the high-speed injection process to operate the plunger such that the plunger moves forward at high speed (S130). Specifically, the controller 40 increases the amount of hydraulic oil to be discharged, by increasing the degree of opening of the servo valve 24. Thus, the plunger 15 connected to the piston 11 moves forward at high speed (e.g., 2 m/sec).

The controller 40 then waits until the position of the plunger 15 moves to the predetermined monitoring range Lw (N in S140). When the plunger 15 moves to the starting position La of the monitoring range Lw (Y in S140), the controller 40 monitors the hydraulic pressure Pf of the front oil chamber 14 on the basis of a signal outputted by the hydraulic pressure sensor 25 until the plunger 15 moves to the ending position Lb of the monitoring range Lw (S150).

The controller 40 then detects that the molten metal M extruded from the sleeve 33 by the forward movement of the plunger 15 has arrived at the cavity (S160). Specifically, the controller 40 waits until the hydraulic pressure Pf of the front oil chamber 14 becomes a lower value than the no-load-time hydraulic pressure P0 by the predetermined percentage α or more (N in S160). When the hydraulic pressure Pf of the front oil chamber 14 becomes a lower value than the no-load-time hydraulic pressure P0 by the predetermined percentage α or more (Pf≤P0×(1−α), Y in S160), as shown in the graph of FIG. 10, the controller 40 determines the then (time Ts) position L of the plunger 15 as the reference position Ls, as shown in FIG. 7. The controller 40 also determines a position ahead of the reference position Ls by the deceleration starting distance D2 as the deceleration starting position L2 and a position ahead of the reference position Ls by the intensification starting distance D3 as the intensification starting position L3 (S170).

The controller 40 then waits until the plunger 15 moves to the deceleration starting position L2 (N in S180). When the plunger 15 moves to the deceleration starting position L2, as shown in FIG. 8 (Y in S180), the controller 40 decelerates the plunger 15 (S190). Specifically, the controller 40 reduces the amount of hydraulic oil to be discharged, by reducing the degree of opening of the servo valve 24. Thus, the controller 40 decelerates the plunger 15 connected to the piston 11.

The controller 40 then waits until the plunger 15 moves to the intensification starting position L3 (N in S120). When the plunger moves to the intensification starting position L3, as shown in FIG. 9 (Y in S200), the controller 40 changes the operation of the plunger from the injection operation to a intensification operation (S210). Specifically, the controller 40 operates the cylinder 12 such that the cylinder 12 moves forward, using the motor-driven intensification mechanism (drive transmission plate 18, electric servo motor 19, and drive transmission mechanism 20) and changes the speed feedback control along the position axis (speed control) to pressure feedback control along the time axis (pressure control). Thus, the controller 40 changes the operation of the plunger 15 from the injection operation to a intensification operation and starts the intensification operation, ending the process of the flowchart. Thereafter, the controller 40 performs the operations in the intensification process, mold opening process, and product ejection process.

As seen above, the die casting machine 1 according to the present embodiment operates the plunger 15 such that the plunger 15 moves forward, detects that the molten metal M extruded from the sleeve 33 by the forward movement of the plunger 15 has arrived at the cavity C, determines the deceleration starting position L2 and intensification starting position L3 for changing the operation of the plunger 15, using, as a reference, the position L (reference position Ls) of the plunger 15 at the time when it is detected that the molten metal M has arrived at the cavity C, and, when the plunger 15 moves to the deceleration starting position L2 or intensification starting position L3, changes the operation of the plunger 15 to an operation corresponding to that position. Since the volume of the cavity C is constant, the amount of molten metal M to be extruded after the molten metal M has arrived at the cavity C is also predetermined. For this reason, the die casting machine 1 determines the position L of the plunger 15 at the time when the molten metal has arrived at the cavity C, as the reference position Ls and determines the deceleration starting position L2 and intensification starting position L3 relative to the reference position Ls, in accordance with the volume of the cavity C. Thus, the die casting machine 1 is able to change the operation of the plunger 15 at an appropriate timing regardless of variations in the amount of molten metal M in the sleeve 33 and thus to effectively stabilize the quality of the product.

The die casting machine 1 includes the piston 11 that is connected to the plunger 15 and operates by hydraulic pressure and the cylinder 12 housing the piston 11. The operation controller 45 of the controller 40 operates the plunger 15 by controlling the supply of the hydraulic oil to the rear oil chamber 13 of the cylinder 12 and the discharge of the hydraulic oil from the front oil chamber 14 of the cylinder 12. The arrival detector 43 of the controller 40 detects that the molten metal M has arrived at the cavity C, on the basis of a change in the hydraulic pressure of the front oil chamber 14. Using such a relatively simple configuration, the die casting machine 1 is able to accurately detect that the molten metal M has arrived at the cavity C.

The arrival detector 43 detects that the molten metal M has arrived at the cavity C, in the monitoring range Lw set with respect to the position L of the plunger 15. By doing so, a position range of the plunger 15 in which the molten metal M is predicted to arrive at the cavity C is set as the monitoring range Lw considering variations in the amount of molten metal M in the sleeve 33. Thus, it is possible to increase the accuracy with which it is detected that the molten metal M has arrived at the cavity C.

When the plunger 15 moves to the deceleration starting position L2, the operation controller 45 decelerates the plunger 15; when the plunger 15 moves to the intensification starting position L3, the operation controller 45 changes the operation of the plunger 15 to an intensification operation. As seen above, the operation controller 45 is able to change the operation of the plunger 15 to the deceleration operation or intensification operation at an appropriate timing.

While, in the above embodiment, when the arrival detector 43 detects that the hydraulic pressure Pf of the front oil chamber 14 has become a lower value than the no-load-time hydraulic pressure P0 of the front oil chamber 14 by the predetermined percentage α or more, it detects that the molten metal M has arrived at the cavity C, this configuration does not have to be used. For example, the arrival detector 43 may detect that the molten metal M has arrived at the cavity C, on the basis of a change in the speed of the plunger 15. Specifically, as schematically shown in FIG. 11, when the arrival detector 43 detects that the speed V of the plunger 15 becomes a lower value than the target speed V0 by a predetermined percentage β or more (V≤V0×(1−β), 0<β<100%), it may detect that the molten metal M has arrived at the cavity C. The reason is as follows: when the top of the molten metal M passes through the gate G, the flow resistance increases and thus a load (pressure) is applied to the plunger 15; and the hydraulic pressure Pf of the front oil chamber 14 is reduced so that the hydraulic pressure Pf and the hydraulic pressure Pb of the rear oil chamber 13 are balanced in the load-applied plunger 15, resulting in a reduction in the speed of the plunger 15. Such a configuration also produces advantageous effects similar to those of the die casting machine 1 of the above embodiment. The configuration in which the arrival detector 43 detects that the molten metal M has arrived at the cavity C may be any configuration without departing from the scope of the present invention.

While, in the above embodiment, the injection operation is performed by hydraulic pressure and the intensification operation is performed by the electric motor, the present invention may be applied to any other configuration, such as one in which the injection operation and the intensification operation are performed by hydraulic pressure or one in which the injection operation and intensification operation are performed by an electric motor.

While the embodiment of the present invention has been described, the present invention is not limited thereto. Addition or deletion of components or a design change on the above embodiment made by those skilled in the art as appropriate and appropriate combinations of the features of the embodiment are also included in the present invention without departing from the scope of the present invention. 

1. A die casting machine comprising: a sleeve into which molten metal to be injected to a cavity of a mold is poured; a plunger housed in the sleeve; an arrival detector configured to detect that molten metal extruded from the sleeve by a forward movement of the plunger has arrived at the cavity; an operation change position determination unit configured to determine an operation change position for changing an operation of the plunger, using, as a reference, a position of the plunger at the time when the arrival detector detects that molten metal has arrived at the cavity; and an operation controller configured to operate the plunger such that the plunger moves forward and to, when the plunger moves to the operation change position, change the operation of the plunger.
 2. The die casting machine of claim 1, further comprising: a piston connected to the plunger and configured to move by hydraulic pressure; and a cylinder housing the piston, wherein the operation controller operates the plunger by controlling supply of hydraulic oil to a rear oil chamber of the cylinder and discharge of hydraulic oil from a front oil chamber of the cylinder, and the arrival detector detects that molten metal has arrived at the cavity, on the basis of a change in hydraulic pressure of the front oil chamber.
 3. The die casting machine of claim 1, wherein the arrival detector detects that molten metal has arrived at the cavity, on the basis of a change in speed of the plunger.
 4. The die casting machine of claim 1, wherein the arrival detector detects that molten metal has arrived at the cavity, within a predetermined monitoring range set with respect to the position of the plunger or the time during which the plunger is operating.
 5. The die casting machine of claim 1, wherein the operation change position is a deceleration starting position, and the operation controller decelerates the plunger when the plunger moves to the deceleration starting position.
 6. The die casting machine of claim 1, wherein the operation change position is an intensification starting position, and the operation controller changes the operation of the plunger to an intensification operation when the plunger moves to the intensification position.
 7. A method for controlling a die casting machine, the die casting machine including a sleeve into which molten metal to be injected to a cavity of a mold is poured and a plunger housed in the sleeve, the method comprising: operating the plunger such that the plunger moves forward; detecting that molten metal extruded from the sleeve by a forward movement of the plunger has arrived at the cavity; determining an operation change position for changing an operation of the plunger, using, as a reference, a position of the plunger at the time when it is detected that molten metal has arrived at the cavity; and when the plunger moves to the operation change position, changing the operation of the plunger. 